# Is a solar system around a black hole possible?

Is it possible for there to be a "solar system" of sorts with a black hole and a few suns orbiting around it?

Could that system be stable and (if so) would it be possible for some civilization(s) to exist on planets orbiting the stars and the black hole?

# Problem 1: The supernova

The first concern I have is one that Zeiss Ikon's answer discusses. To form a black hole, you need some sort of energetic event, likely a supernova. However, a supernova releases three extremely problematic sources of energy:

• High-energy photons, like gamma rays, that have the potential to strip away the atmosphere of any pre-existing planets in the vicinity - or, at the very least, to remove their ozone layers.
• Several solar masses worth of hot, fast-moving ejecta (think something on the order of $$\sim$$10,000 km/s, heating up any nearby gas as it travels outwards. Again, not super awesome for planets and their atmospheres.
• A flood of neutrinos, carrying away the bulk of the explosion's energy. They're really not phenomenal. That said, I'm unsure of how neutrino heating in an atmosphere would go.

Plus, this supernova's progenitor was probably a massive star, and massive stars have strong stellar winds (on the order of $$\sim$$1000-2000 km/s), which also have the potential to ablate atmospheres.

We might be able to form a black hole without a supernova, via a situation called a failed supernova. The idea is that a sudden emission of neutrinos prior to core collapse could carry away enough to mass-energy to substantially reduce the luminous energy output of the explosion. The collapse would still create a burst of energy, but it might be much less deadly than a normal supernova, perhaps even preserving the planet's atmosphere, if it already existed.

Now, we haven't gotten rid of the whole neutrino problem; in fact, we've increased it. However, again, perhaps the energy transfer isn't as intense as I think it might be. I'll need to do some reading.

# Problem 2: Fun with orbits

You also have to consider that the orbits of the system could get pretty funky. Asymmetry in the explosion could create an effect similar to a pulsar kick, propelling the black hole at several hundred kilometers per second. If it traveled slow enough to remain gravitationally bound to the system, it might be traveling in a fairly elliptical orbit, and if you want to have multiple other stars, you'd have to worry about their orbits being disrupted.

Additionally, plenty of mass is lost during a supernova - most of the progenitor's mass, in many cases. This, too, will disrupt the orbits. It would be interesting to model this to see exactly what would happen, and if the effects would indeed be problematic. Systems with three or more stars are already kinda sensitive to dramatic enough perturbations - and believe me, this mass-loss would be quite the perturbation!

One possible solution would be for the planet-hosting star and the black hole to come together after the supernova occurs - in other words, for one to gravitationally capture the other. This requires a third body to mediate the interaction - so, for instance, if the black hole interacted with a binary star, one of the stars would have to be ejected for the other to become bound to the black hole.

This, of course, presents more orbital difficulties. A three-body encounter would likely disrupt the planet's orbit, if it had already formed. If it formed after the encounter - well, that demands explanation. Perhaps it formed from the debris disk left behind by the progenitor star. But then why would it orbit the secondary star, not the black hole? That, as far as I can see, remains a problem. Capturing the planet, as Cadence suggested, might be a way around it, but the orbital dynamics would be . . . delicate.

• An early black hole might catch a forming star system and merge to a SMBH / main sequence star binary. That way you'd avoid supernova and still can have a long burning yellow main sequence star to have a world on it develop life. Still the SMBH and the star would have to be far enough away to allow for a stable planetary orbit around the star. – Adwaenyth Dec 3 '18 at 15:24
• Could the planet be captured after the black hole/star system is already established? Or would any reasonable origin for the planet still be too close to the supernova? – Cadence Dec 3 '18 at 15:30
• @Cadence That's a reasonable possibility, yes. I would assume it could happen. – HDE 226868 Dec 3 '18 at 16:42
• Regarding an outburst of neutrinos: what-if.xkcd.com/73. I think if a large portion of the energy from a failed supernova was emitted as neutrinos it would still be catastrophic for the surrounding system. – bob0the0mighty Dec 3 '18 at 17:59
• I'm pretty sure that a black hole can gravity-deform a star enough for capture without a third body. – Joshua Dec 3 '18 at 18:17

Yes.

You can take binary or trinary star systems and swap one of the stars for a black hole and nothing changes in the orbital dynamics.

Depending on the layout of the solar system planets can orbit the stars, the black hole, or some mixture of the above.

Some of those planets could be in the habitable zone (liquid water).

And some of those planets could develop intelligent life.

We don't know what the probability of most of thees steps is (especially the last one) but we know they are all plausible.

• Given that the only method we know to form a stellar-mass black hole is a supernova, other stars in a multiple-star system with the black hole would have any planets sterilized by the supernova explosion -- formation of life on their planets would then have to start over from zero. – Zeiss Ikon Dec 3 '18 at 13:56
• @ZeissIkon And have millions of years to do so...or the black hole could be captured later or formed through a different mechanism we're current unaware of. Once formed the system would be stable plenty long enough for life to evolve though. – Tim B Dec 3 '18 at 14:07
• Of course, this is all providing the supernova doesn't just strip away their atmospheres. There's concern that flare activity from M-type red dwarves can do that to their close-in planets. Could a supernova do it in a binary/trinary/etc. system? I don't know. – Zeiss Ikon Dec 3 '18 at 14:21
• @ZeissIkon The atmosphere would be a problem. The system once it forms is stable though, it would be a much bigger Q&A to also try and look at how it could form although that would be an interesting question in its own right. – Tim B Dec 3 '18 at 15:08
• It's also worth mentioning that gravitational capture can be a mechanism to explain the development of a multiple-star system, long after the supernova event. – Beanluc Dec 3 '18 at 19:35

Maybe.

The only mechanisms we know that can form a stellar mass black hole (a few to a few tens of times the mass of our sun) are neutron star mergers (seemingly very rare) and supernova explosions. Either of these are extremely energetic events, and since planets apparently form as part of stellar formation, all the planets in the system will exist at the time of the supernova that created the black hole (neutron stars also form in supernovae, but the time for two to decay orbits and merge would probably exceed the viable lifetime of a habitable planet).

A supernova within a few light years, never mind in the same gravity bound star system, would likely sterilize any existing planets; one within a fraction of a light year (say, even a 500 year orbit) would probably strip the atmosphere from a rocky planet, almost certainly ruining it for future re-evolution of life. Such an event would very certainly destroy any existing civilization that couldn't flee well ahead of time.

You'd need a situation where a planet manages to at least retain an atmosphere (and liquid water or other suitable liquid solvent, say liquid hydrocarbons as on Titan) for life to have a chance to re-evolve, or even for a planet to be suitable for recolonization after the black hole forms. No, not impossible -- the universe is a big place -- but pretty unlikely.

• This answer and a couple others talk about how it would only be possible if a planet somehow retained its atmosphere after a supernova event. What about if the planet had its atmosphere stripped but somehow it was later replenished (naturally or artificially)? – Abion47 Dec 3 '18 at 21:37
• @Abion47 As I understand current theory, the atmosphere forms with the planet; there's no mechanism for a rocky planet to reconstitute a habitable atmosphere short of an event like our own Moon-forming collision that melts the planet to the core -- and those are vanishingly rare by the time the parent star has reached the main sequence. – Zeiss Ikon Dec 4 '18 at 12:04
• I had the same thought, that the event I could think of that could replenish a rocky planet's atmosphere after a supernova stripped it off would be an asteroid or comet collision of epic proportions. These would certainly be rare, but would they be less likely to occur than the rocky planet somehow retaining its atmosphere in spite of the supernova? I just don't know enough to be certain. – Abion47 Dec 4 '18 at 16:40
• You'd need bigger than an asteroid. Thea (the object that struck Earth to create our comically oversized Moon) was about as big as Mars is now, and the one that probably struck Mars was roughly as big as our Moon. And these collisions were four+ billion years ago. – Zeiss Ikon Dec 4 '18 at 17:38

The situation might evolve in a globular cluster where stars ar fairly close together. IF a black hole in moving through the cluster at the right velocity, it might attract stars into an orbital arrangement with the black hole in the centre, and the planetary systems of the stars remaining in orbit around them.

This is going to be a delicate arrangement, since the act of gravitationally attracting the star to the black hole is likely to cause disruptions in the orbits of the planets, kicking planets out of habitable zones, or alternatively moving them into habitable zones.

The likely look of such a system from far enough away will be similar to a comet, with the "tail" being billions of comets and Oort cloud objects from the various star systems, followed by some ejected planets, then the stars and finally the black hole itself, likely visible by the accretion disc made up of the relatively denser dust and interstellar gas from the globular cluster.

Imagine the bright nucleus is the accretion disc of the black hole, and the "tail" is the objects scattered and pulled along by the black hole. The star systems with planets would be about 1/3 of the way back from the nucleus, any closer and they would be roasted by the energy of the accretion disc

The sky from these planets will be interesting. On a planet you still have your sun and other planets (likely in rather eccentric orbits), with several other stars nearby and an amazingly bright object high above the plane of the ecliptic. The constellations will change rapidly (say over the lifetime of a civilization), constellations visible in the early Roman Empire will be long gone by the collapse of the Empire because of the orbit of the stars around the black hole.

If mankind created an artificial black hole that eats earth, then it would have the mass of earth. It would be in the same orbit as earth was and there shouldn't be any big change.

So maybe if a terrible accident happened to a colonized planet or if the greatest mass murder of all history happened during an interplanetary war, then I guess it could.

• @mongo if that was a general agreement, there wouldn't have been so much panic over the LHC some years ago. – Renan Dec 3 '18 at 17:36
• @mongo Presumably you are referring to the likeliness of the black hole forming in the first place, and not to a physical constraint on the size? (if the latter, then I would have to disagree) – JBentley Dec 3 '18 at 17:53
• @Renan: The "panic" was from laypeople who didn't know better, and such black holes would be formed by different mechanisms anyway. Black holes of stellar origin must be greater than about 2-5 solar masses (not 25). It's physically possible to form less massive black holes, but not from gravitational collapse (you'll just get a neutron star or white dwarf instead). en.wikipedia.org/wiki/Black_hole#Gravitational_collapse – Will Vousden Dec 3 '18 at 17:58
• With regards to that 25-solar mass figure, see my comment below - mongo's figure refers to the mass of the black hole progenitor, not the black hole itself. Regardless of the number, this figure - by no means a limit - is for black holes produced by stellar collapse, rather than artificial methods. – HDE 226868 Dec 3 '18 at 18:03
• @HDE226868: There's a lower mass limit for core collapse black holes. If you could force drive the creation by some other means, that limit would not exist for the other means. – Joshua Dec 4 '18 at 4:10

It is not only possible, it is normal: Earth is a planet orbiting a star which is orbiting the black hole at the center of our galaxy.

• Minor nitpick: Center of our galaxy. – Zan Lynx Dec 4 '18 at 16:46
• This isn't quite true. We're orbiting the center of the galaxy, and a supermassive black hole happens to be at the center, but it's much less massive than the galaxy as a whole. In terms of the motion of the Sun, it's insignificant. See also worldbuilding.stackexchange.com/q/32900/627. – HDE 226868 Dec 4 '18 at 17:07
• @HDE226868, you make it sound like it is a complete coincidence that there just happens to be a black hole in the center of our galaxy. I think it is more likely that it is there precisely because mass collects in the center and forms a black hole. So the black hole is necessarily there. – hkBst Dec 4 '18 at 17:16
• @hkBst It's not a coincidence, no, but we're not really orbiting the black hole. Using that word implies that the gravitational force from the black hole is the reason for the Sun's orbit, which isn't true. Drawing an analogy between a galaxy and the sort of system the OP wants is kinda inaccurate; the dynamics are much different. – HDE 226868 Dec 4 '18 at 17:18

A black hole, the likely size to act as a central mass, with orbiting suns, would likely be the result of a supernovae, which would have a typical lower size of about 25 solar masses. The nearby few suns would likely have great orbital distances and/or high orbital velocities. Eventually the black hole will increase in gravitational draw, and then slowly the suns would be sucked in. In other words, this scenario is an unstable one. Additionally, it would involve extremely large orbital distances and velocities. Planets orbiting would have to have high orbital velocities, and like the suns might be subject to substantial tidal forces. Higher orbital velocities incur a higher probability of matter accumulation, which is incompatible with the stability to support life.

So not very likely.

Addendum: Stated differently, the probabilities for "solar systems" of multiple stars, referencing the original poster term of "a few suns" orbiting around a black hole, carries the implication of a larger black hole, and since the observed black holes are in the 10 to 100 solar mass region, with 25 being near the modal for the distribution (and perhaps for other reasons as well), the larger solar mass black holes would permit "orbiting around" of a few suns. Secondly, in the Milky Way it is estimated that most stars, perhaps 85 percent, are red dwarfs, which are generally about 0.2 solar mass. Additionally, estimates (some are less) of the red dwarfs have stellar companions. So to have a black hole, with an orbiting set of a few suns orbiting it, appears to be a rather improbable event, given our contemporary understanding of stellar demographics. However, things do not end there.

To have a system of planets orbiting the back hole with a few suns orbiting it stellar collection, would likely require large orbital distances and/or high orbital speeds, as the central mass would be rather high. That is, not the tiniest black hole, and then again a few suns in orbit. So we are talking a large footprint for the resulting solar system.

The original poster asks if civilizations, which I suppose means "life as we know it" to be inhabiting one or more of those planets. Now the collection of those few suns and the black hole must be distributed in a rather uniform manner to permit somewhat uniform insolation of the possible planet in the green zone. The probabilities keep dwindling.

So we have a mid-sized black hole, with a few suns, somewhat uniformly orbiting it, and those suns should not be of the ever popular red dwarf type, and somehow this arrangement will provide somewhat uniform insolation to a planet sitting in the green zone of the system.

COULD something like this exist? Who could say definitely yes or no? However the majority of solar systems are binary, so that tips the scale towards systems of more than one central star. However, the effects of multiple solar mass bodies tends to have other effects which enter into consideration.

Accretion rates, are likely to cause a doubling of mass from tidally disrupted stars (2x10^10 years approx), and for gravitational diffusion (2x10^9 years approx). There is much more to factor in, but the doubling of the black hole mass within the lifetime of a planet such as earth is real, and would have disastrous effects. For example, the luminous flux of the resulting suns would be compromised, changing drastically the green zone. Additionally the ever cycling tidal forces of the multiple stars would likely create a substantial fluidity of the planet structures. This might also affect the development of civilization.

A few suns in addition to having the right irradiance, would also have to have rather regular orbits, as eccentricity would adversely impact consistent planetary insolation.

So summarizing this, a central mass black hole, orbited by a few suns, with sufficient and uniform insolance on a planet in a green zone, which would be presumably free from excessive tidal forces, and having that central mass black hole be stable and not substantially accumulating mass from accretion or other effects in what promises to be a region ripe with tidal forces and gravitational diffusion, seems rather unlikely.

• Hi, mongo. Can you provide a citation for the claim that "A black hole has a minimum size of about 25 solar masses"? Many, if not most, of the known stellar-mass black holes are much less massive; M33 X-7 is near the top, and it's only about $15M_{\odot}$. – HDE 226868 Dec 3 '18 at 16:42
• It has been quite some time since I studied this stuff, and I remember the 25 solar mass part, but I agree that there are black holes in the range of 4 solar masses. So your question is fair, and spaces.imperial.edu/russell.lavery/ASTR100/Lectures/… makes reference to the creation of black holes from supernovae, citing the creation of a back hole at 25 solar masses. I suppose plank's defines the smallest possible black hole, but the identification of those is not common. – mongo Dec 3 '18 at 16:58
• It would be interesting to load up a STK simulation with the proposed masses and distances, and see just how stable the system would be. However, because the back hole will tend to accumulate more mass over time (than a sun) it seems that the system would be unstable. – mongo Dec 3 '18 at 17:12
• Ah, you're confusing the mass of the black hole progenitor - the star that collapses - and the mass of the black hole itself. Much of that mass is lost, either via ejecta, neutrinos, or simple energy from supernova nucleosynthesis. The mass of a stellar-mass black hole is much, much less than that of its progenitor. – HDE 226868 Dec 3 '18 at 18:00
• You can also get a black hole by using neuron stars. The upper limit before collapse is around 3.2 solar masses. – Draco18s no longer trusts SE Dec 3 '18 at 18:39